The present invention relates to fluid pressure control devices for vehicle braking systems, and more particularly to a improvement of a fluid pressure control device of the type which comprises a housing provided thereon with an inlet port for connection to a master cylinder and an outlet port for connection to wheel brake cylinders and provided therein with a stepped bore in communication with the inlet and outlet ports respectively at the small and large diameter portions thereof and a first fluid chamber provided in parallel with the stepped bore and in communication with the small and large diameter portions of the stepped bore, a differential piston slidably disposed within the stepped bore to provide second and third fluid chambers respectively in open communication with the inlet and outlet ports, a spring loading the piston axially in a direction toward the outlet port, and a cut-off valve assembly including a valve seat provided on the side wall of the first chamber to permit fluid flow between the second and third fluid chambers and an inertia-controlled ball housed within the first chamber to co-operate with the valve seat and rolling toward the valve seat to cut-off fluid flow from the second chamber to the third chamber when the ball is subjected to a deceleration in excess of a predetermined value.
In the conventional fluid pressure control device of this kind, the stepped bore for the differential piston and the first fluid chamber for the cut-off valve assembly are arranged in parallel within the housing to minimize the whole size of the control device. In use of such a control device, the parallel arrangement of the stepped bore and the first fluid chamber causes the air to remain within the second and third fluid chambers which are formed at both sides of the stepped bore. The air also remains in the upper portion of the first fluid chamber since the inertia-controlled ball is housed in the first fluid chamber and the communication between the first and third fluid chambers is conducted through the valve seat. The air remained within the fluid chambers gets into the braking fluid applied to the wheel brake cylinders to harm the braking effects.